1. Field of the Invention
The present invention relates to a sheet conveying device that conveys a sheet discharged from an image forming apparatus, a print system, and a sheet cooling method.
2. Description of the Related Art
In image forming apparatuses using electrophotography, when a sheet on which a toner image is transferred passes through a fixing unit, toner transferred onto the sheet is fused and is tightly contact with the sheet so as to be fixed thereto. Although the sheet is conveyed while being naturally cooled after the sheet passes through the fixing unit, the surface temperature of the sheet continues to be high after the sheet passes through the fixing unit.
The toner fixed to the sheet has adherence until it is cooled. Accordingly, under an undesirable condition, when the sheets are loaded in a discharging unit, a phenomenon called a toner blocking phenomenon occurs, such as the adhesion of toner to a sheet face on which loaded sheets overlap each other or toner peel-off at the time of separating sheets.
The undesired condition refers to a case where sheets are consecutively conveyed without any intermission, a case where a large quantity of sheets is loaded in the loading unit (a case where the airtightness at the time of overlapping sheets is high or a case where there are many sheets loaded into the loading unit so that pressure is applied to the sheets), a case where toner images are fixed to both faces of a sheet, a case where the fixing temperature is set to be high, a case where the environmental temperature or the environmental humidity is high, a case where a sheet of a type that has a low degree of toner fixation is used, a case where sheets are loaded in the loading unit for a long time, or the like.
A. In order to prevent such a blocking phenomenon, various means for cooling sheets have been devised until now. FIG. 1 is a diagram illustrating an overview of a conventional cooling conveyance device. The cooling conveyance device includes: a conveying path that is arranged on the downstream of an image forming process and has a “U” shape for cooling sheets; air intake fans (the top face and the bottom face) that take in the external air; and an air exhaust fan (the rear face of the cooling conveyance device when viewed from the surface of paper) that discharges the air inside the cooling conveyance device to the outside of the device.
A sheet conveyed from the image forming apparatus is conveyed while being in contact with a metal plate arranged in the conveying path for cooling, whereby the temperature thereof decreases. The metal plate, which is arranged in the conveying path of the cooling conveyance device and comes to bear heat due to the passage of the sheet, is designed so as to be cooled by receiving a wind blowing from the air intake fan and maintain the cooling effect of the sheet. In other words, as long as the sheets are conveyed one by one with a sufficient gap maintained therebetween, the metal plate is cooled by the wind generated by the air intake fan, and accordingly, the metal plate can cool the sheets to the extent that the toner blocking phenomenon does not occur.
In addition, on the downstream of the cooling conveyance device, a stacker is arranged so that the sheets can be stacked there.
B. In addition, in order to suppress the toner blocking phenomenon of the loaded sheets, a cooling method is proposed in which a sheet is cooled by temporarily stopping or decelerating the sheet on the conveying path of the cooling device (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2007-079151).
C. Furthermore, for the purpose of cooling a conveyed sheet, a cooling method is proposed in which two lines of conveying paths for duplex printing are included, and the conveyed sheet is cooled by alternately using the conveying paths (for example, see JP-A No. 2009-265349). According to such a conventional technique, in a case where a sheet heated by a fixing unit is supplied to the image forming unit in a state of not being sufficiently cooled, the temperature of toner disposed inside the developing unit increases due to heat dissipated from the sheet so that the chargeability degrades and an image abnormality such as a thin image density occurs. Accordingly, in order to prevent such an image abnormality, the sheet, which has passed the fixing unit, is cooled in the conveying path, the cooled sheet is conveyed to a transfer unit again, toner is transferred to the sheet, and then the sheet passes through the fixing unit.
However, according to the cooling method represented in A, in a case where the above-described condition is extremely, or a plurality of the above conditions is set simultaneously, when the sheets are conveyed sequentially, heat gradually accumulates in the metal plate disposed in the conveying path, and accordingly, the cooling effect may be degraded. Accordingly, there is a problem in that the toner blocking phenomenon of loaded sheets may occur. A case may be considered in which the conveyance interval of sheets may be set to be wide enough to maintain the cooling effect. However, in such a method, since it is difficult to sequentially convey the sheets, the productivity decreases.
In addition, according to the cooling method represented in B, although it is sure that the sheets can be cooled easily, it is difficult to sequentially convey the sheets in the cooling conveyance method in which the sheet is stopped or decelerated. Accordingly there is a problem in that the productivity decreases.
Furthermore, according to the cooling method represented in C, the sheet can be cooled during a period until the rear face is printed after the front face is printed. However, after the rear face is printed, the sheet does not pass through the conveying path that is used for cooling. Accordingly, the sheet with a high temperature is conveyed to the loading unit, and therefore, there is a problem in that the toner blocking phenomenon of loaded sheets occurs.
Besides the methods represented in A, B, and C, further technique may be considered in which two conveying paths having different lengths are arranged in a cooling conveyance device, and a sheet is conveyed along a longer conveying path under a condition that the toner blocking phenomenon may easily occur. However, a way of merely using a long conveying path may lead to the decreased cooling capability of the metal plate soon as sheets are sequentially conveyed, and hence the toner blocking phenomenon may occur.